Signal amplification by illumination of a partially developed latent electrostatic image

ABSTRACT

Method and apparatus for reducing X-ray dosage required for xeroradiographic examinations without reducing the relative information capacity of the images produced during the examination. In particular, a charged xerographic plate is positioned adjacent the object to be examined and penetrating Xray radiation is projected through the object onto the plate surface, forming a latent electrostatic image on the surface of the plate. The penetrating radiation utilized is of a substantially lower dosage than normally utilized. The image is then partially developed with developing powder and the partially developed image is then exposed to substantially uniform radiation. The exposed image is finally developed by applying additional developing powder thereto resulting in an enhanced image or signal.

United States Patent Jeromin 1451 Oct. 21, 1975 Primary ExaminerJames.W. Lawrence Assistant Examiner-D, C. Nelms IMAGE Attorney, Agent, or Firm-James J. Ralabate; Terry J. Anderson; Irving Keschner [75] Inventor: Lothar S. Jeromln, S1erra Madre,

Calif.

[73] Assignee: Xerox Corporation, Stamford, [57] ABSTRACT Conn. Method and apparatus for reducing X-ray dosage re- [22] Flledi 4, 1974 quired for xeroradiographic examinations without re- [211 App] 448,128 ducing the relative information capacity of the images produced during the examination. In particular, a charged xerographic plate is positioned adjacent the [52] 250/315 A; R; 355/3 R object to be examined and penetrating X-ray radiation Cl- "F is projected through the object nto the plate surfa 1 Fleld of 250/315 A; 355/3 forming a latent electrostatic image on the surface of 355/16, R the plate. The penetrating radiation utilized is of a substantially lower dosage than normally utilized. The References Cited image is then" partially developed with developing UNITED STATES PATENTS powder and the partially developed image is then ex- 2,711,481 6/1955 Phillips 250/315 A posed to Substantially uniform radiation The exposed 2,756,676 7/1956 Steinhilper,, %/1 image is finally developed by applying additional de- 3,l46,100 8/1964 Kaufman 96/1 veloping powder thereto resulting in an enhanced 3,603,790. 9/1971 Cleare 355/17 image or signal, 3,776,627 12/1973 Ohnishi et al.... 355/3 3,830,645 8/1974 Zwe1g 96/1 13 Claims, 7 Drawing Figures FOREIGN PATENTS OR APPLICATIONS 812,419 4/1959 United Kingdom 96/1 64 62 P X- RAY SOURCE H VOLTAGE I I I F 32 L 3 d 38 3/ E 33 40 i I a a I HIGH I ctouo VOLTAGE i GENERATOR SOURCE 5 U.S. Patent Oct. 21, 1975 Sheet 2 of5 3,914,609.

I6OOV-- I rX 05f ROENTGEN HIGH b VI/I/II/IIA l. A 22 34 k 9 a5 I :I I

38 33 40 W VHIGH CLOUD VOLTAGE GENERATOR SOURCE FIG. 4

US. Patent Oct. 21, 1975 Sheet 4 of5 3,914,609

w m w\\ vmw wmw Ill/E U.S. Patent 0a. 21, 1975 Sheet 5 of5 SIGNAL AMPLIFICATION BY ILLUMINATION OF A PARTIALLY DEVELOPEDLATENT ELECTROSTATIC IMAGE BACKGROUND OF THE INVENTION Xeroradiography, as disclosed in U.S. Pat. No. 2,666,144, is a process wherein an object is internally examined by subjecting the object to penetrating radiation. A uniform electrostatic charge is deposited on the surface of a xerographic plate, and a latent electrostatic image is created by projecting the penetrating radiation such as X-rays or gamma rays through the object and onto the plate surface. The latent electrostatic image may be made visible by contacting the latentelectrostatic image on the plate surface with fine powdered particles electrically charged opposite to the latent electrostatic pattern on the plate. The visible image may be viewed, photographed or transferred to another surface where it may be permanently affixed or otherwise utilized. The entire processing is dry, and no dark room is necessary.

Xeroradiography in recent years has been utilized to detect breast cancer in women. In the examination of breasts wherein soft tissue comprises most of the breast area, Xeroradiography or xeromammography, as it is generally called, provides greater resolving power than the conventional roentgenographic film, and greater image detail is achieved. A wide range of contrast is seen on the xerographic plate as compared to the conventional roentgenographic films so that all structures of the breast from the skin to the chest wall and ribs may be readily visualized. Besides providing better contrast, xeromammography detects small structures like tumor calcification and magnifies them more than conventional film, is quicker, less expensive, gives greater detail and requires less radiation than prior nonphotoconductive X-ray techniques.

A factor which has influenced some radiologists to limit Xeroradiography applications to the examination of breasts and the extremities (i.e., hands and feet) is the radiation dosage required in examining chests, skulls, hips, etc. as compared to conventional screened X-ray film.

Since a substantial portion of the generated X-ray radiation will remain in the body tissues of the target area, radiologists have been reluctant to use X-ray systems requiring radiation dosage levels which, although below the recognized minimum safety level, is still SUMMARY OF THE PRESENTINVENTION The present invention provides method and appara- 'tus for reducing the X-ray dosage required for xeroradiographic examinations without reducing the relative information capacity of the images produced during the examination. In particular, a charged xerographic plate is positioned adjacent the object to be examined and penetrating radiation, such as X-rays, are

projected through the object onto the plate surface, forming a latent electrostatic image on the surface of the plate. The penetrating radiation utilized is of a lower dosage than normally utilized. The image is then partially developed with developing powder and the partially developed image is then exposed to substantially uniform radiation. The exposed image is finally developed by applying additional developer powder thereto resulting in an enhanced image or signal.

It is an object of the present invention to provide method and apparatus for reducing the X-ray dosage required for object examinations in a xeroradiographic system.

It is a further object of the present invention to provide method and apparatus for reducing the X-ray dosage required for object examinations in Xeroradiographic systems wherein the latent electrostatic image, after exposure to a reduced dosage of X-rays, is partially developed, exposed to uniform radiation and then finally developed to produce an enhanced image or signal having the equivalent relative information capacity of an image produced by utilizing a higher dosage of X- rays.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, as well as other objects and further features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawings wherein:

FIG. 1 is a representation of the voltage differences and charge patterns formed on the surface of a photoreceptor in accordance with one embodiment of the present invention;

FIG. 2 is an exposure curve illustrating the charge retention capabilities of a photoreceptor after exposure to X-rays;

FIG. 3 is a representation of the voltage differences and charge patterns formed on the surface of a photoreceptor in accordance with another embodiment of the present invention;

FIG. 4 shows a diagrammatic cross-section of apparatus which may be utilized to implement the teachings of the present invention;

FIG. 5 is a schematic view of an automated xerographic processing system which may be utilized to implement the teachings of the present invention; and

FIGS. 6a and 6b illustrate in schematic form the implementation of the light scanning step in the system described with reference to FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, representations (a) through (d) illustrate, in schematical form, one embodiment of the present invention and, in particular, a technique for enhancing the signal level of a positively developed xeroradiographic image. The left-hand portion of the representations illustrate the voltage levels, Ve, of the electrostatic charge pattern formed on the surface of a xeroradiographic plate as a function of a distance X along the plate whereas the corresponding right-hand portion of the representations illustrate the charge Q (and developer powder deposition) on the plate surface as a function of distance X along the plate surface.

A uniform X-ray beam 10 is generated by a conventional X-ray source 12 and transmitted through an object 14. The X-ray beam is attenuated by the object 14, the subject of the examination procedure, and the transmitted X-rays from the object are modulated in accordance with the density of the object 14 generating a charge pattern of varying surface potentials on the surface of initially positively charged photoreceptor, or xeroradiographic plate, 16.

The difference in surface potential between two areas is referred to as a signal, and for certain small, low density objects, the exposure is optimal when the differences in surface potential (signal) are very large. This is the case, in general, when the average image surface potential falls into the center of the steepest slope of the exposure curve. Any lower or higher exposure for this particular object reduces the signal strength, i.e., reduces the relative information capacity of the image. FIG. 2 illustrates a typical exposure curve wherein the surface potential Ve remaining on the surface of the photoreceptor after exposure is plotted with respect to the roentgen dose r of the exposure beam. As can be observed from the graph, the steepest part of the curve is at approximately /2 Ve and 0.05 r and corresponds to the optimal surface potential for optimal signal strength.

FIG. 1 describes'the invention wherein positive images are formed on the surface of the photoreceptor. In other words, the image, when developed, will be a positive of the object being examined.

The first step of the technique shownat (a) is to expose a charged xeroradiographic plate to the X-rays transmitted by object 14, the surface of the xeroradiographic plate being initially charged, for example, to 1,600 volts. Assuming that the object 14 has a uniform absorption characteristic, the resulting representation is a voltage step, corresponding to the X dimension of the object 14, of a magnitude AVe those areas of the photoreceptor surface under the object are not discharged whereas the remaining areas of the plate, corresponding to the width of the X-ray beam, are substantially discharged. In the representation illustrated, the remaining plate areas are discharged to approximately onehalf of the initial surface potential, i.e., 800 volts. It should be noted that various techniques can be utilized to charge the surface of the xeroradiographic plate 16 including the well known use of corona generators..

To describe the relative savings in X-ray dosage required, if it is assumed, for example, that the object 14 to be examined is a human chest, an incident dosage of l Roentgen (l r), corresponding to an X-ray source operation at 120 Kvp and 100 mAs would be required by present xeroradiographic techniques. However, in the technique of the present invention, a reduction in the roentgen dose of approximately two to four times can be realized. In the present example, the incident roentgen dosage is reduced by a factor of two, corresponding to 120 Kvp and 50 mAs. The latent electrostatic charge is then partially developed by applying developer powder to the surface of the photoreceptor. As shown at (b), the developer powder, or toner particles, charged negatively in this illustration since the plate was initially charged positive, is deposited denser in the areas of higher surface charge (corresponding to object 14) than in the areas of lower surface charge, the potential difference AVe on the surface of the xeroradiographic plate being less than the potential difference at the first step of the process shown in (a), for the reason that more developer powder is deposited on the higher charged area which effectively reduces the surface potential of the chargepattern, i.e., the developer powder (toner) partially neutralizes the surface charge. In the example illustrated, the surface potential corresponding to the denser areas of the image is reduced to approximately 1,590 volts, corresponding to a potential difference of 790 volts between the areas of higher and lower surface charge.

In the next step of the present invention, the surface of the photoreceptor, having the partially developed image thereon (which forms an opaque mask on the photoreceptor surface), is exposed uniformly to a light source such as that generated by a standard fluorescent lamp which generates light in the visible spectrum. Since the wavelength of the generated illumination should be compatible with the sensitivity of the photoreceptor, appropriate light filtering is preferably utilized. For example, for the photoreceptor described in the aforementioned U.S. Pat. No. 3,650,620, comprising a conductive backing member having coated on a portion of one surface thereof a photoconductive insulating coating such as vitreous selemium, a blue filter is preferably interposed between the light source and the surface of the photoconductive coating. The light produced by the lamp discharges the lighter developed areas more than the darker developed area due to the ability of the powder particles to absorb light. The pow der particles therefore should have the capability of absorbing light emitted by the aforementioned lamp.

As shown at (c), the voltage difference, Ave on the photoreceptor surface after light exposure is greater than AVe and results in a charge and developer powder distribution as shown on the right-hand portion of (0). Typically, the surface potential corresponding to the higher initial charge has been reduced to 1,550 volts, whereas the surface potential corresponding to the lower initial charge has been reduced to substantially zero volts, a signal of approximately 1,550 volts. The uniform light exposure can be accomplished by moving the photoreceptor past a stationary lamp or by moving the lamp across the photoreceptor surface.

After the light exposure, the photoreceptor is then subjected to an additional development step whereby the developer powder particles are again applied to the photoreceptor surface, resulting in the representations shown at (d). As can be observed, additional developer powder has been attracted to the positive charge on the photoreceptor surface, reducing the voltage difference, AVe,, to a magnitude less than Ave, but greater than AVe 'Typically, the surface potential corresponding to the higher initial surface charge has 1,540 reduced to 1540 volts, whereas the surface potential corresponding to the lowerinitial charge remains at substantially zero volts. The voltage difference Ave, provides an image which has a relative information capacity (i.e., 1,540 volt signal) which is substantially equivalent to an image which would be developed if the full X-ray dosage was utilized for exposure. In the latter situation, full exposure would reduce those areas of theinitially charged photoreceptor subjected to the penetrating radiation to substantially zero volts, providing an image which has a signal of i -l ,600 volts.

As will be described in more detail in FIG. 4, the photoreceptor substrate is preferably maintained at apositive bias in order to ensure that the negatively charged developer powder particles are attracted to the positive ages. As with reference to FIG. 1, object 14 is initially exposed to penetrating X-rays l0 generated by X-ray source 12 (the same reference numerals are utilized in the figures to identify similar elements), the dosage being lower than normally required, resulting in a charge pattern (and corresponding voltage difference AVe,) on the surface of photoreceptor 16 (photoreceptor 16 comprising a photoconductive insulating layer 18, such as vitreous selenium, and conductive substrate as shown at (a). As described hereinabove with reference to FIG. 1, the charge density is greater on those portions of the photoreceptor which receive the least X-ray radiation. The latent electrostatic charge pattern is then subjected to an initial development wherein a negative bias greater than the maximum positive sur face potential (AVe is applied to the photoreceptor substrate. Hence, positively charged developer powder particles are deposited denser in the areas of lower surface charge than in the areas of higher surface charge (background development), resulting in the charge and developer powder distribution as shown at the right hand portion of (b). The corresponding potential difference, AVe is less than AVe due to the greater charge deposition in the areas of lower surface charge. A more detailed discussion of the effectof substrate bias on positive-negative development, edge deletion and image contrast control is set forth in copending application Ser. No. 323,666, filed Jan. 15, 1973, and assigned to the assignee of the present invention. The surface of the photoreceptor 18 is then uniformly exposed to light, resulting in the representations shown at (c). The charge pattern formed on the photoreceptor surface is substantially discharged in the image areas since the thickness of the layer of powder particles is less than the thickness of the layer of powder particles'in the background areas resulting in a pattern wherein only charged background areas are present. The voltage difference, or signal AVe is greater than AVe and greater than the original signal Ave,

In the final step shown at (d), the remaining background charge pattern is developed with negatively charged developer powder, the bias on the photorecep tor substrate being at ground or slightly positive, which produces a negatively developed image having substantially the same relative information capacity of an image produced by utilizing the full X-ray dosage which normally would have been utilized to develop an image of object 14.

The voltage difference at the photoreceptor surface, AVe is approximately 1,160 volts which corresponds favorably to the voltage difference (signal) of 1,600 volts obtained at full exposure.

In order to reduce the granularity of the negative image produced, the surface of the photoconductive layer 18 may be slightly AC charged after initial development.

It should be further noted that the developer powder used for final development may be of a different color than the developer powder utilized during partial development to produce better image contrast. For example, a blue developer powder can be used for the partial development whereas a black developer powder may be utilized for final development.

Referring now to FIG. 4, apparatus which may be utilized to implement the technique of the present invention is illustrated. In particular, a xerographic plate composed of layer 18 overlying conductive member 20 is placed on supports 22, a source of biasing potential 23 being applied to conductive member 20. The plate is sensitized or charged by passing across the surface of layer 18 corona discharge electrode 30 preferably comprising one or several fine conductive strands supplied with a corona-generating voltage from high voltage source 31. Door 32 in a side of chamber 34 is adapted to open to allow corona discharge electrode 30 to enter chamber 34. Electrode 30 is driven by conventional drive means while supported and positioned by guide means and when operated will pass in front of and across the surface of plate layer 18 to place thereon a uniform electrostatic charge while conductive member 20, is at a positive potential. A sliding member 33 is positioned within one side of chamber 34 and formed to completely enclose powder cloud storing area 35 and separate it from chamber 34 when conventional means are utilized for pulling the sliding member across the lower portion of chamber 34. When the system is operative, sliding member 33 is released to allow it to rewind upon itself due to a spring controlled recoil action creating one open area composed of powder cloud storing area- 35 and chamber 34. A vacuum cleaner 36 is positioned to allow vacuum cleaner nozzle 36 to extend through a side in chamber 34. Cloud spray nozzle 40 extends into cloud storing area 35 from a cloud generator with powder particles mixed in pressurized air. Nozzle 40 has an internal opening through which the particles supplied in pressurized air are supplied to the cloud storing area 35. In the positive mode of developconventional drive means, such as a motor driven lead screw, in a scanning mode in front of and across the surface of photoconductive layer 18. g

In operation, an object to be examined may be placed on conductive member 20, conductive member 20 acting as a support for the object 60 to be examined. Door 32 opens to the position shown and corona electrode 30 is caused to traverse across the surface of layer 18 supplying charge thereto. At the end of the charging pass door 32 closes. An exposure is then made by causing penetrating radiation generated by X-ray source 62 to be directed to and through object 60, source 62 being energized by high voltage supply 64. During initial exposure, sliding member 33 remains in a closed position while a cloud of charged developer particles are supplied to storage area 35. Following exposure, sliding member 33 is released making storage area 35 and chamber 33 one enclosure. The cloud of developer particles in air which is already generated and which is continuously being generated during development is supplied to the plate following exposure by allowing the powder cloud to flow upward into the area of influence of the electrostatic latent image on the surface of layer 18. After the proper development time has elapsed, sliding member 33 returns to its closed position and vacuum cleaner 30 may be operated to purge the remaining cloud of particles in air from chamber 34. An air intake valve may also be opened to prevent the creation of a negative pressure within chamber 34. After initial development, the light source 50 is caused to traverse the photoconductive layer 18 in a scanning mode of operation and to expose layer 18 to illumination of a proper wavelength as explained hereinabove. After the scanning operation, light source 50 is caused to return to its initial position by conventional means. After the light exposure, sliding member 33 is again released and the cloud of developer particles is supplied again to the plate surface, thereby finally developing the image. Sliding member 33 returns to its closed position and the purge cycle is repeated. At this point the plate is ready for viewing and/or further processing to obtain a permanent record of the image and the object is removed to give access to the plate. This cycle of operation which has just been described may then be repeated. I

The plate used in connection with this invention may be a conventional plate used in the art of xeroradiography as set forth hereinabove and includes plates generally used in xerographic apparatus. Backing member 12 may be any conductive material, a preferred plate, however, having a backing member composed of aluminum or aluminum having a radiation absorbant coating thereon such as a coating of lead. Other metals or conductors operate very well in this invention when used as backing members in properly formed xeroradiographic plates. Layer 16 should be composed of a manal exposure station. The aforementioned processing system may be adapted to incorporate the novel techniques of the present invention with a simple modification thereto as will be described hereinafter. In order to place the present invention in proper perspective relative to the aforementioned automated processing system, theoperation thereof will be briefly described.

Storage box 80, is inserted into charging unit 90 through port 92. A- xerographic plate 94, with the photoconductor layer on the top side, is withdrawn there from and passed to conditioning means 96 where it is maintained at the appropriate temperature for a predeterial which becomes conductive when exposed to penetrating radiation and which in the absence of penetrating radiation is a good insulator. Typical materials which may be employed in accordance with the present The operation described hereinabove'would be iden tical to the negative, or background, mode of development, with the modification of having an initial negative bias on the conductive member 20 greater than the initial positive surface potential and changing the bias to ground or slightly positive at the final development step (see discussion set forth hereinabove). Further, in order to reduce granularity of the negative image, the surface of layer 18 may be slightly AC charged after partial development.

Referring to FIG. 5, there is shown a schematic illustration of the operative elements of the automated, flatplate xerographic processing system described in U.S. Pat. No. 3,650,620 showing the relationship of two automated processing units to each other and to an exter' termined period of time whereby the residual image normally associated with the exposure of xerographic plates to high energy penetrating radiation, such as X- rays, is eliminated. After the predetermined conditioning period, the xerographic plate is withdrawn from the conditioning means 96 and passed to storage magazine 100 where i t is cooled to the proper xerographic pro.-

cessing temperature by means of air drawn about the xerographic plate by cooling fan 102. In accordance with operating conditions described in the aforementioned patent, upon insertion of an empty cassette 104 through port 106 in charging unit 90, xerographic plate 94 is withdrawn from storage magazine 100, passed beneath vacuum cleaning means 110 and uniform electrostatic charging means 42 and into cassette 104, which is automatically released and closed whereby the uniformly charged xerographic plate is held in a lighttight environment.

Upon withdrawal of the plate-bearing cassette from the charging unit, it is taken to the external X-ray exposure station, properly positioned with respect to the radiation source and the object being examined, and ex posed to imaging radiation of a reduced dosage in acj cordance with the teachings of the present invention.'

Thereafter, the cassette, with the latent electrostatic image-bearing xerographic plate therein, is inverted and inserted into printing unit 200 through port 204. If

the operating conditions are met, the cassette is automatically opened and the xerographic plate, held in proper alignment with the xerographic plate processing path by the internal structure of the cassette, is withdrawn and transported to powder cloud development. means 210. During development, a single support sheet is withdrawn from support sheet supply means 212. This sheet is transported by transport means 214 to a point adjacent the path of xerographic plate travel during its advancement from the development chamber, where the sheet is stopped. After initial development, the development chamber is lowered in a manner described in the aforementioned'patent and scanning means 216, comprising lamp 218 and housing 220 having an aperture slit therein, is caused to pass across and adjacent to the surface of the plate 10. It should be noted at this point that appropriate bias levels for posi-. tive and negative. images are controlled by biasing agrid interposed between the plate surface and a baffle in lieu of applying a bias directly to the plate substrate.

negative development is concerned, by appropriate biasing of the plate substrate. This latter named development chamber is, as described in the application, easily adapted for use in the automated flatplate xerographic processing system described in U.S. Pat. No. 3,650,620. After the scanning means 216 is returned to its initial position, the development chamber is raised to engage the plate 10 is an air sealed environment and final development is then initiated. Although the details of the drive mechanism for scanning means 216 and the modification to the timing sequence disclosed in U.S. Pat. No. 3,650,620 as a result thereof have not been shown, it is believed that these machine modifications are within the capabilities of one skilled in the electromechanical arts. After the final development step, the xerographic plate, with the powder image thereon, is transported out of the development means, over pretransfer corotron 224 which uniformly charges the photoconductive surface and the powder image to a first polarity. As the leading edge of the xerographic plate comes into registration with the stationary support sheet, they are caused to move in synchronization over transfer corotron 226 which charges the back side of the support-sheet to a polarity opposite the charging polarity utilized by pre-transfer corotron 224, whereby the powder image is transferred to the support sheet. Continued movement of the xerographic plate in synchronization with the underlying support sheet causes the support sheet with the toner image thereon to come in contact with gripper bar transport assembly 228 which strips the support sheet from its position adjacent the xerographic plate and transports the support sheet to fuser means 230 where the powder image is permanently bonded to the support sheet surface. Continued rotation of belt transport means 232 within the fuser means causes the ejection of the xerographic copy into receiving tray 234.

After the support sheet has been stripped from its position adjacent the xerographic plate, the plate passes over pre-clean corotron 236 and into contact with brush cleaner 238 which removes residual toner from the photoconductive surface. The movement of the plate is continued into inverted storage box 240. To I complete the cycle, it is only necessary to withdraw storage box 240 from printing unit 300 through port 242. Invert the storage box such that slot 244 is in the lower left-hand corner, and insert the storage'box into charging unit 90 through port 92. In this manner, the xerographic plates can be reused for subsequent xerographic processing.

'The apparatus described in reference to FIG. can be utilized to implement the negative development mode by appropriate control of the bias applied to the grid (or plate substrate if the development chamber described in the aforementioned copending application is utilized). Further, in order to reduce granularity of the negative image, the surface of the photoreceptor may be slightly. LAC charged after initial development.

FIGS; 6d and6b illustrate in schema'ticalform the light scanning step of the present invention utilizing the automated flat-plate processing system described with reference to FIG. 5.

After the plate "10 is positioned above development chamber 210, the development chamber is raised into contact with plate 10 by inflatable means 250 (FIG. 6a) and the image is partially developed by applying approximately seven bursts of toner to the chamber. After initial (partial) development, chamber2l'0 is'-lowered 65 tion of arrow 264, the light generated from lamp 218 being emitted from aperture slit 221. After the initial scan, the scanning means returns in the direction of arrow 266 to its initial, or home, position. After scan- 5 ning means 216 reaches its initial position, the development chamber is raised (position shown in FIG. 6a) and finally development occurs by applying approximately ten bursts of toner. After final development, the chamber is lowered and the normal process continues.

[0 The development chamber shown in FIGS. 6a and 6b is described in detail in U.S. Pat. No. 3,640,246. It is to be recalled that the development chamber described in the aforementioned copending application may be utilized with appropriate modification since the plate is raised or lowered onto the development chamber.

It should be noted that two development chambers, one for initial or partial development, and the other for final development, can be utilized in lieu of the single development chamber as described hereinabove.

If it is desired to utilize a different colored toner for the initial and final developmentsteps to increase image contrast, a second toner dispenser may be added. Altemately, a single toner dispenser with two separate compartments, each having a separate aspirator tube, can be utilized.

The system described hereinabove may be arranged wherein the scanning means is maintained outside the development chamber in a fixed position and the plate transported across the scanning means aperture slit after initial development, the plate thereafter being returned to the chamber for final development.

While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teaching of the invention without de- 40 parting from its essential teachings.

face comprising the steps of:

a. providing a photoconductive surface having at least two adjacent initial charge patterns of differing charge density thereon, said charge patterns being of a first polarity, said photoconductive surface being sensitive to light of a predetermined wavelength,

initially depositing a developer'powder which is charged to a second polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of higher surface charge density than in the areas of lower surface charge density, thereby providing a developed charge pattern,

c. uniformly exposing the developed charge pattern of light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and

d. thereafter applying additional developer powder charged to said second polarity to said developed charge pattern whereby the potential difference between adjacent developed charge patterns of differing density is at least equal to the potential dif ference between said adjacent initial charge patterns.

2. A method of increasing the potential difference between two adjacent developed charge patterns of differing charge density formed on a photoconductive surface comprising the steps of:

a. providing a photoconductive surface having at least two adjacent initial charge patterns of differing charge density thereon, said charge patterns being of a first polarity, said photoconductive surface being sensitive to light of a predetermined wavelength, Y

b. initially depositing a developer powder which is charged to said first polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of lower surface charge density than in the areas of higher surface charge density, thereby providing a developed charge pattern, 1

c. uniformly exposing the developed charge pattern to light of said predetermined wavelength, the initially deposited developer powder forming a mask 7 which absorbs light in proportion to the density of the deposited developer powder, and

d. thereafter applying additional developer powder charged to a second polarity to said developed charge pattern whereby the potential difference between adjacent developed charge patterns of differing density is at least equal to the potential difference between said adjacent initial charge patterns.

3. A method of increasing the potential difference between two adjacent developed latent electrostatic charge patterns of differing charge density formed on a photoconductive surface, said latent electrostatic charge patterns being produced by positioning an object to be imaged adjacentsaid photoconductive surface and exposing said object to penetrating radiation comprising the steps of:

a. providing a charged photoconductive surface adjacent an object to be imaged, said photoconductive surface being sensitive to light of a first wavelength,

b; passing penetrating radiation through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface whereby at least two adjacent initial charge patterns of differing charge density is formed on said photoconductive surface, said charge patterns being of a first polarity,

initially depositing a developer powder which is charged to a second polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of higher surface charge density than in the areas of lower surface charge density, thereby providing a developed image, uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and

thereafter applying additional developer powder charged to said second polarity to said developed image whereby the potential difference between 6 4. A method of increasing'the potential difference between two adjacent developed latent electrostatic charge patterns of differingcharge density formed on a photoconductive surface, said latent electrostatic charge patterns being produced by positioning an object to be imaged adjacent said photoconductive surface and exposing said object to penetrating radiation comprising the steps of:

a. providing a charged photoconductive surface adjacent an object to be imaged, said photoconductive surface being sensitive to light of a predetermined wavelength,

b. passing penetrating radiation through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface whereby at least two adjacent initial charge patterns of differing charge density is'formed on said photoconductive surface, said charge patterns being of a first polarity,

. initially depositing a developer powder which is charged to said first polarity on said photoconductive surface whereby said developer powder is de' posited in the areas of lower surface charge density than in the areas of higher surface charge density, thereby providing a developed image,

d. uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and

. thereafter applying additional developer powder charged to a second polarity to said developed image whereby the potential difference between adjacent charge patterns of differing density is at least equal to the potential difierence between said adjacent initial charge patterns.

5. The method as defined in claim 4 further including the step of AC charging said photoconductive surface prior to depositing developer powder charged to said first polarity.

6. Apparatus for increasing the potential difference between two adjacent developed charge patterns of differing charge density formed on a photoconductive surface comprising:

a. a photoconductive surface having at least two adjacent initial charge patterns of differing charge density thereon, said charge patterns being of a first polarity, said photoconductive surface being sensitive to light of a predetermined wavelength,

b. means for initially depositing a developer powder which ischarged to a second polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of higher surface charge density than in the areas of lower surface charge density, thereby providing a developed 'charge pattern,

means for unifonnly exposing the developed charge pattern to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and i d. means for applying additional developer powder cha'rgedfto said second polarity to said developed charge pattern whereby the potential difference between "adjacent charge patternslof differing density is at least equal to the potential difference between said adjacent initial charge patterns.

7. Apparatus for increasing the potential difference between two adjacent developed charge patterns of differing charge density formed on a photoconductive surface comprising:

a. a photoconductive surface having at least two adjacent initial charge patterns of differing charge density thereon, said charge patterns being of a first polarity, said photoconductive surface being sensitive to light of a predetermined wavelength,

b. means for initially depositing a developer powder which is charged to said first polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of lower surface charge density than in the areas of higher surface charge density, thereby providing a developed charge pattern,

c, means uniformly for exposing the developed charge pattern to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and

(1. means for applying additional developer powder charged to a second polarity to-said developed charge pattern whereby the potential difference between adjacent charge patterns of differing density is at least equal to the potential difference between said adjacent initial charge patterns.

8. Apparatus for increasing the potential difference between two adjacent developed latent electrostatic charge patterns of differing charge density formed on a photoconductive surface, said latent electrostatic charge patterns being produced by positioning an object to be imaged adjacent said photoconductive surface and exposing said object to penetrating radiation comprising:

a. a charged photoconductive surface adjacent an object to be imaged, said photoconductive surface being sensitive to light of a predetermined wavelength,

b. means for generating penetrating radiation, said penetrating radiation passing through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface, at least two adjacent initial charge patterns of differing charge density being formed on said photoconductive surface, said charge patterns being of a first polarity,

0. means for initially depositing a developer powder which is charged to a second polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas ofhigher surface charge density than in the areas of lower surface charge density, thereby providing a developed image, I

d. means for uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and

- e. means for applying additional developer powder charged to said second polarity to said developed image whereby the potential difference between adjacent charge patterns of differing density is at least equal to the potential difierence between said adjacent initial charge patterns.

9. Apparatus for increasing the potential difference between two adjacent latent electrostatic charge patterns of differing charge density formed on a photoconductive surface, said latent electrostatic charge patterns being produced by positioning an object to be imaged adjacent said photoconductive surface and exposing said object to penetrating radiation comprising:

a. a charged photoconductive surface adjacent an object to be imaged, said photoconductive surface being sensitive to light of a predetermined wavelength,

b. means for generating penetrating radiation, said penetrating radiation passing through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface, at least two adjacent initial charge patterns of differing charge density being formed on said photoconductive surface, said charge patterns being of a first polarity,

. means for initially depositing a developer powder which is charged to said first polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of lower surface charge density than in the areas of higher surface charge density, thereby providing a developed image,

. means for uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and

e. means for applying additional developer powder charged to a second polarity to said developed image whereby the potential difference between adjacent charge patterns of differing density is at least equal to the potential difference between said adjacent initial charge patterns.

10. A method of forming a xeroradiographic image of an object by exposing the object to a first dosage of penetrating radiation, the formed xeroradiographic image having a relative information content which is equivalent to that of a xeroradiographic image formed by exposing the object to penetrating radiation of second dosage, said first dosage being lower than said second dosage, comprising the steps of:

a. providing a photoconductive surface having a substantially uniform charge pattern thereon, said photoconductive surface being sensitive to light of a predetermined wavelength,

b. positioning an object to be imaged adjacent said photoconductive surface,

c. passing penetrating radiation of said first dosage through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface whereby at least two adjacent charge patterns of differing charge density is formed on said photoconductive surface, said charge patterns being of a first polarity,

d. initially depositing a developer powder which is charged to a second polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of higher surface charge density than in the areas of lower surface charge density, thereby providing a developed image,

e. uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and

f. thereafter applying additional developer powder charged to said second polarity to said developed image whereby the relative information content of the additionally developed image is equivalent to that of an image formed by exposing the object to penetrating radiation of said second dosage.

11. A method of forming a xeroradiographic image of an object by exposing the object to a first dosage of penetrating radiation, the formed xeroradiographic image having a relative information content which is equivalent to that of a xeroradiographic image formed by exposing the object to penetrating radiation of second dosage, said first dosage being less than said first dosage, comprising the steps of:

a. providing a photoconductive surface having a substantially uniform charge pattern thereon, said photoconductive surface being sensitive to light of a predetermined wavelength,

b. positioning an object to be imaged adjacent said photoconductive surface,

0. passing penetrating radiation through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface whereby at least two adjacent charge patterns of differing charge density is formed on said photoconductive surface, said charge patterns being of a first polarity,

initially depositing a developer powder which is charged to said first polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of lower surface charge density than in the areas of higher surface charge density, thereby providing a developed image,

e. uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and

f. thereafter applying additional developer powder charged to a second polarity to said developed image whereby the relative information content of the additionally developed image is equivalent to that of an image formed by exposing the object to penetrating radiation of said second dosage.

12. Apparatus for forming a xeroradiograhic image of an object by exposing the object to a first dosage of penetrating radiation, the formed xeroradiographic image having a relative information content which is equivalent to that of a xeroradiographic image formed by exposing the object to penetrating radiation of a second dosage, said first dosage being lower than said second dosage, comprising:

a. a charged photoconductive surface positioned adjacent an object to be imaged, said photoconductive surface being sensitive to light of a predeter mined wavelength, b. means for generating penetrating radiation of said first dosage, said penetrating radiation passing through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface, at least two adjacent charge patterns of differing charge density being formed on said photoconductive surface, said charge patterns being of a first polarity,

. means for initially depositing a developer powder which is charged to a second polarity on said photoconductivesurface whereby said developer powder is deposited denser in the areas of higher surface charge density than in the areas of lower surface charge density, thereby providing a developed image,

d. means for uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and

e. means for applying additional developer powder charged to said second polarity to said developed image whereby the relative information content of the additionally developed image is equivalent to that of an image formed by exposing the object to penetrating radiation of said second dosage.

13. Apparatus for forming a xeroradiographic image of an object by exposing the object to a first dosage of penetrating radiation, the formed xeroradiographic image having a relative information content which is equivalent to that of a xeroradiographic image formed by exposing the object topenetrating radiation of a second dosage, said first dosage being lower than said second dosage, comprising:

a. a charged photoconductive surface positioned adjacent an object to be imaged, said photoconductive surface being sensitive to light of a predetermined wavelength,

b. means for generating penetrating radiation of said first dosage, said penetrating radiation passing through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface, at least two adjacent charge patterns of differing charge density being formed on said photoconductive surface, said charge patterns being of a first polarity,

. means for initially depositing a developer powder which is charged to said first polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of lower surface charge density than in the areas of higher surface charge density, thereby providing a developed image,

d. means for uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and e.- means for applying additional developer powder charged to a second polarity to said developed image whereby the relative information content of the additionally developed image is equivalent to that of an image formed by exposing the object to penetrating radiation of said second dosage. It: l 

1. A method of increasing the potential difference between two adjacent developed charge patterns of differing charge density formed on a photoconductive surface comprising the steps of: a. providing a photoconductive surface having at least two adjacent initial charge patterns of differing charge density thereon, said charge patterns being of a first polarity, said photoconductive surface being sensitive to light of a predetermined wavelength, b. initially depositing a developer powder which is charged to a second polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of higher surface charge density than in the areas of lower surface charge density, thereby providing a developed charge pattern, c. uniformly exposing the developed charge pattern of light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and d. thereafter applying additional developer powder charged to said second polarity to said developed charge pattern whereby the potential difference between adjacent developed charge patterns of differing density is at least equal to the potential difference between said adjacent initial charge patterns.
 2. A method of increasing the potential difference between two adjacent developed charge patterns of differing charge density formed on a photoconductive surface comprising the steps of: a. providing a photoconductive surface having at least two adjacent initial charge patterns of differing charge density thereon, said charge patterns being of a first polarity, said photoconductive surface being sensitive to light of a predetermined wavelength, b. initially depositing a developer powder which is charged to said first polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of lower surface charge density than in the areas of higher surface charge density, thereby providing a developed charge pattern, c. uniformly exposing the developed charge pattern to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and d. thereafter applying additional developer powder charged to a second polarity to said developed charge pattern whereby the potential difference between adjacent developed charge patterns of differing density is at least equal to the potential difference between said adjacent initial charge patterns.
 3. A method of increasing the potential difference between two adjacent developed latent electrostatic charge patterns of differing charge density formed on a photoconductive surface, said latent electrostatic charge patterns being produced by positioning an object to be imaged adjacent said photoconductive surface and exposing said object to penetrating radiation comprising the steps of: a. providing a charged photoconductive surface adjacent an object to be imaged, said photoconductive surface being sensitive to light of a first wavelength, b. passing penetrating radiation through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface whereby at least two adjacent initial charge patterns of differing charge density is formed on said photoconductive surface, said charge patterns being of a first polarity, c. initially depositing a developer powder which is charged to a second polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of higher surface charge density than in the areas of lower surface charge density, thereby providing a developed image, d. uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and e. thereafter applying additional developer powder charged to said second polarity to said developed image whereby the potential difference between adjacent developed charge patterns of differing density is at least equal to the potential difference between said adjacent initial charge patterns.
 4. A method of increasing the potential difference between two adjacent developed latent electrostatic charge patterns of differing charge density formed on a photoconductive surface, said latent electrostatic charge patterns being produced by positioning an object to be imaged adjacent said photoconductive surface and exposing said object to penetrating radiation comprising the steps of: a. providing a charged photoconductive surface adjacent an object to be imageD, said photoconductive surface being sensitive to light of a predetermined wavelength, b. passing penetrating radiation through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface whereby at least two adjacent initial charge patterns of differing charge density is formed on said photoconductive surface, said charge patterns being of a first polarity, c. initially depositing a developer powder which is charged to said first polarity on said photoconductive surface whereby said developer powder is deposited in the areas of lower surface charge density than in the areas of higher surface charge density, thereby providing a developed image, d. uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and e. thereafter applying additional developer powder charged to a second polarity to said developed image whereby the potential difference between adjacent charge patterns of differing density is at least equal to the potential difference between said adjacent initial charge patterns.
 5. The method as defined in claim 4 further including the step of AC charging said photoconductive surface prior to depositing developer powder charged to said first polarity.
 6. Apparatus for increasing the potential difference between two adjacent developed charge patterns of differing charge density formed on a photoconductive surface comprising: a. a photoconductive surface having at least two adjacent initial charge patterns of differing charge density thereon, said charge patterns being of a first polarity, said photoconductive surface being sensitive to light of a predetermined wavelength, b. means for initially depositing a developer powder which is charged to a second polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of higher surface charge density than in the areas of lower surface charge density, thereby providing a developed charge pattern, c. means for uniformly exposing the developed charge pattern to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and d. means for applying additional developer powder charged to said second polarity to said developed charge pattern whereby the potential difference between adjacent charge patterns of differing density is at least equal to the potential difference between said adjacent initial charge patterns.
 7. Apparatus for increasing the potential difference between two adjacent developed charge patterns of differing charge density formed on a photoconductive surface comprising: a. a photoconductive surface having at least two adjacent initial charge patterns of differing charge density thereon, said charge patterns being of a first polarity, said photoconductive surface being sensitive to light of a predetermined wavelength, b. means for initially depositing a developer powder which is charged to said first polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of lower surface charge density than in the areas of higher surface charge density, thereby providing a developed charge pattern, c. means uniformly for exposing the developed charge pattern to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and d. means for applying additional developer powder charged to a second polarity to said developed charge pattern whereby the potential difference between adjacent charge patterns of differing density is at least equal to the potential difference between said adjacent initial charge paTterns.
 8. Apparatus for increasing the potential difference between two adjacent developed latent electrostatic charge patterns of differing charge density formed on a photoconductive surface, said latent electrostatic charge patterns being produced by positioning an object to be imaged adjacent said photoconductive surface and exposing said object to penetrating radiation comprising: a. a charged photoconductive surface adjacent an object to be imaged, said photoconductive surface being sensitive to light of a predetermined wavelength, b. means for generating penetrating radiation, said penetrating radiation passing through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface, at least two adjacent initial charge patterns of differing charge density being formed on said photoconductive surface, said charge patterns being of a first polarity, c. means for initially depositing a developer powder which is charged to a second polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of higher surface charge density than in the areas of lower surface charge density, thereby providing a developed image, d. means for uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and e. means for applying additional developer powder charged to said second polarity to said developed image whereby the potential difference between adjacent charge patterns of differing density is at least equal to the potential difference between said adjacent initial charge patterns.
 9. Apparatus for increasing the potential difference between two adjacent latent electrostatic charge patterns of differing charge density formed on a photoconductive surface, said latent electrostatic charge patterns being produced by positioning an object to be imaged adjacent said photoconductive surface and exposing said object to penetrating radiation comprising: a. a charged photoconductive surface adjacent an object to be imaged, said photoconductive surface being sensitive to light of a predetermined wavelength, b. means for generating penetrating radiation, said penetrating radiation passing through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface, at least two adjacent initial charge patterns of differing charge density being formed on said photoconductive surface, said charge patterns being of a first polarity, c. means for initially depositing a developer powder which is charged to said first polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of lower surface charge density than in the areas of higher surface charge density, thereby providing a developed image, d. means for uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and e. means for applying additional developer powder charged to a second polarity to said developed image whereby the potential difference between adjacent charge patterns of differing density is at least equal to the potential difference between said adjacent initial charge patterns.
 10. A method of forming a xeroradiographic image of an object by exposing the object to a first dosage of penetrating radiation, the formed xeroradiographic image having a relative information content which is equivalent to that of a xeroradiographic image formed by exposing the object to penetrating radiation of second dosage, said first dosage being lower than said second dosage, comprising the steps of: a. providing a photoconductive surface having a substantially uniform cHarge pattern thereon, said photoconductive surface being sensitive to light of a predetermined wavelength, b. positioning an object to be imaged adjacent said photoconductive surface, c. passing penetrating radiation of said first dosage through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface whereby at least two adjacent charge patterns of differing charge density is formed on said photoconductive surface, said charge patterns being of a first polarity, d. initially depositing a developer powder which is charged to a second polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of higher surface charge density than in the areas of lower surface charge density, thereby providing a developed image, e. uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and f. thereafter applying additional developer powder charged to said second polarity to said developed image whereby the relative information content of the additionally developed image is equivalent to that of an image formed by exposing the object to penetrating radiation of said second dosage.
 11. A method of forming a xeroradiographic image of an object by exposing the object to a first dosage of penetrating radiation, the formed xeroradiographic image having a relative information content which is equivalent to that of a xeroradiographic image formed by exposing the object to penetrating radiation of second dosage, said first dosage being less than said first dosage, comprising the steps of: a. providing a photoconductive surface having a substantially uniform charge pattern thereon, said photoconductive surface being sensitive to light of a predetermined wavelength, b. positioning an object to be imaged adjacent said photoconductive surface, c. passing penetrating radiation through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface whereby at least two adjacent charge patterns of differing charge density is formed on said photoconductive surface, said charge patterns being of a first polarity, d. initially depositing a developer powder which is charged to said first polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of lower surface charge density than in the areas of higher surface charge density, thereby providing a developed image, e. uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and f. thereafter applying additional developer powder charged to a second polarity to said developed image whereby the relative information content of the additionally developed image is equivalent to that of an image formed by exposing the object to penetrating radiation of said second dosage.
 12. Apparatus for forming a xeroradiograhic image of an object by exposing the object to a first dosage of penetrating radiation, the formed xeroradiographic image having a relative information content which is equivalent to that of a xeroradiographic image formed by exposing the object to penetrating radiation of a second dosage, said first dosage being lower than said second dosage, comprising: a. a charged photoconductive surface positioned adjacent an object to be imaged, said photoconductive surface being sensitive to light of a predetermined wavelength, b. means for generating penetrating radiation of said first dosage, said penetrating radiation passing through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said sUrface, at least two adjacent charge patterns of differing charge density being formed on said photoconductive surface, said charge patterns being of a first polarity, c. means for initially depositing a developer powder which is charged to a second polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of higher surface charge density than in the areas of lower surface charge density, thereby providing a developed image, d. means for uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and e. means for applying additional developer powder charged to said second polarity to said developed image whereby the relative information content of the additionally developed image is equivalent to that of an image formed by exposing the object to penetrating radiation of said second dosage.
 13. Apparatus for forming a xeroradiographic image of an object by exposing the object to a first dosage of penetrating radiation, the formed xeroradiographic image having a relative information content which is equivalent to that of a xeroradiographic image formed by exposing the object to penetrating radiation of a second dosage, said first dosage being lower than said second dosage, comprising: a. a charged photoconductive surface positioned adjacent an object to be imaged, said photoconductive surface being sensitive to light of a predetermined wavelength, b. means for generating penetrating radiation of said first dosage, said penetrating radiation passing through said object and onto said charged photoconductive surface whereby an electrostatic image of said object is formed on said surface, at least two adjacent charge patterns of differing charge density being formed on said photoconductive surface, said charge patterns being of a first polarity, c. means for initially depositing a developer powder which is charged to said first polarity on said photoconductive surface whereby said developer powder is deposited denser in the areas of lower surface charge density than in the areas of higher surface charge density, thereby providing a developed image, d. means for uniformly exposing the developed image to light of said predetermined wavelength, the initially deposited developer powder forming a mask which absorbs light in proportion to the density of the deposited developer powder, and e. means for applying additional developer powder charged to a second polarity to said developed image whereby the relative information content of the additionally developed image is equivalent to that of an image formed by exposing the object to penetrating radiation of said second dosage. 